Apparatus for mixing and applying reactive flowable materials, for example polyurethane starting compounds, in reverse coating, are known. In one design a high-towered pronged stirrer is fitted into the mixing chamber. This apparatus is very prone to breakdown. The seal of the material bush usually starts leaking after only one to three hours, product escapes and the mixing ratio changes, thus impairing the quality of the product. Another known apparatus makes use of a complicated rotor with distributing shovels (German Offenlegungsschrift No. 2,530,018).
If a hot product is also to be processed, then, owing to the different expansion in heat, the problems of sealing can no longer be solved.
Stirrers of the types described for example in German Offenlegungsschriften Nos. 2,451,367 (U.S. Pat. Nos. 3,923,202 and 3,982,668) or 2,645,876 (U.S. Pat. No. 4,049,197) or in German Auslegeschriften Nos. 2,530,018 or 2,035,485 are unsuitable for this purpose.
External mixing nozzles, whirl chambers, counter-flow injectors and static mixers are types of mixing apparatus without moving parts.
With external mixing nozzles, the material to be mixed is not sufficient (German Offenlegungsschrift No. 1,570,524 and U.S. Pat. No. 3,475,266).
With whirl chamber mixing (for example German Offenlegungsschrift No. 2,262,739 and U.S. Pat. No. 3,862,907) it is not possible to obtain lasting, large amounts of material to be mixed with media having very different viscosities, since the material to be mixed is greatly impaired by deposits in the chamber. A very large amount of air is used up, bubbles are difficult to avoid in the surface layer and losses through over-spraying impair the economic viability.
With mixing according to the counter-flow injection principle (German Offenlegungsschrift No. 1,694,151), high pressures are needed and complicated designs are required. This method is particularly disadvantageous when high-viscosity and low-viscosity media are processed together.
If only a static mixer (German Auslegeschrift No. 1,557,118 or German Offenlegungsschrift No. 2,525,020, U.S. Pat. No. 4,019,719) is used, the mixing chamber has to be made relatively long. The residence time, therefore, becomes too great for rapidly reacting components. The same applies to a valve for mixing two flowing agents of differing viscosity (German Auslegeschrift No. 2,216,871).
An object of the invention is to develop a method and an apparatus for mixing and applying rapidly reacting materials with large differences in viscosity, and possibly differences in temperature, which is distinguished by good reliability of operation.